Cyclopentadienyl manganese compounds are excellent antiknocks in gasoline used to operate spark ignited internal combustion engines. Use of such compounds as antiknocks is described in U.S. Pat. No. 2,818,417; U.S. Pat. No. 2,839,552 and U.S. Pat. No. 3,127,351, incorporated herein by reference. Not only are these compounds effective antiknock agents, but it has also been found that they do not adversely affect the activity of catalysts used to decrease the amount of undesirable constituents in engine exhaust gas. Under some operating conditions it has been found that, although the manganese antiknocks do not lessen the activity of the exhaust gas catalyst, they can interact in some manner at the surface of the catalyst bed leading to a reduction in the size of the openings into the bed thereby causing an increase in exhaust back pressure. The present invention provides a simple effective means of alleviating this problem.